Fuel cells which generate electric current by controllably combining elemental hydrogen and oxygen are well known. In one form of such a fuel cell, an anodic layer and a cathodic layer are separated by a permeable electrolyte formed of a ceramic solid oxide. Such a fuel cell is known in the art as a “solid oxide fuel cell” (SOFC). Hydrogen, either pure or reformed from hydrocarbons, is flowed along the outer surface of the anode and diffuses into the anode. Oxygen, typically from air, is flowed along the outer surface of the cathode and diffuses into the cathode. Each O2 molecule is split and reduced to two O−2 ions catalytically by the cathode. The oxygen ions diffuse through the electrolyte and combine at the anode/electrolyte interface with four hydrogen ions to form two molecules of water. The anode and the cathode are connected externally through the load to complete the circuit whereby four electrons are transferred from the anode to the cathode. When hydrogen is derived by “reforming” hydrocarbons such as gasoline in the presence of limited oxygen, the “reformate” gas includes CO which is converted to CO2 at the anode. Reformed gasoline is a commonly used fuel in automotive fuel cell applications.
A single cell is capable of generating a relatively small voltage and wattage, typically between about 0.5 volt and about 1.0 volt, depending upon load, and less than about 2 watts per cm2 of cell surface. Therefore, in practice it is known to stack together, in electrical series, a plurality of cells. Because each anode and cathode must have a free space for passage of gas over its surface, the cells are separated by perimeter spacers which are selectively vented to permit flow of gas to the anodes and cathodes as desired but which form seals on their axial surfaces to prevent gas leakage from the sides of the stack. The perimeter spacers may include dielectric layers to insulate the interconnects from each other. Adjacent cells are connected electrically by “interconnect” elements in the stack, the outer surfaces of the anodes and cathodes being electrically connected to their respective interconnects by electrical contacts disposed within the gas-flow space. In the prior art, such electrical contacts are formed typically by a metallic foam which is readily gas-permeable or by conductive filaments. The outermost, or end, interconnects of the stack define electric terminals, or “current collectors,” which may be connected across a load.
It can be difficult in using metallic foam or conductive filaments to control the axial loading between adjacent fuel cell modules. The gas flow spaces may be easily deformed during assembly of a stack, through deformation of anodes, cathodes, and or interconnects. Any such deformation affects the flow path of hydrogen and air and therefore the electrical performance of cells and the overall stack.
What is needed is an improved mechanical means for defining and maintaining the size and shape of the gas flow spaces in a fuel cell stack while also providing electrical contact between the surfaces of the electrodes and their respective interconnect elements.
It is a principal object of the present invention to provide reliable and durable electrical contact between the surfaces of the electrodes and their respective interconnect elements in a fuel cell stack.
It is a further object of the invention to provide such electrical contact while maintaining the size and shape of the gas flow spaces in a fuel cell stack.
It is a still further object of the invention to provide means for mechanical support of the fuel cell when it is subjected to thermally induced stress and vehicle vibration.
It is a still further object of the invention to provide means for influencing the flow of gases through the gas flow spaces to more evenly distribute gases over the surfaces of the electrodes and thereby improve the electric output and fuel efficiency of a fuel cell assembly.